Gli proteins constitute a vital but complex group of transcription factors. Humans have three, and mutations in them or errors in their regulation can cause developmental disorders or cancer. Our understanding of these proteins has rested on studies of their Drosophila ortholog Cubitus interruptus, which revealed that Gli proteins are a major target of the Hedgehog-signaling pathway, and can be processed to produce either activators or repressors. However, key questions about Gli function remain unanswered. In particular, it is not clear how much of their activity is controlled by co-factors, or what role they play in epigenetic changes. Since Gli proteins are broad transcriptional regulators, these questions are critical. Caenorhabditis nematodes provide an ideal model for answering these questions. They have a single Gli protein, TRA-1, that shares many similarities with human Gli proteins. However, TRA-1 controls sexual fates and plays a central role in self-fertility. As a result, the balance between its activating and repressing functions in the germ line makes C. briggsae ideal for identifying and evaluating co-factors. Finally, the anatomy of C. briggsae and C. elegans are simple and completely defined, which makes it feasible to study tissue-specific effects of chromatin regulation during development. Similar studies are difficult in other animals. The power of nematode genetics and developmental biology will simply the identification of Gli co-factors, and the analysis of how they work with TRA-1 to activate or repress targets. Indeed, direct transcriptional control and epigenetic effects can both be studied in living animals. Thus, this proposal has three aims: Aim #1: Define TRA-1 activator and repressor functions in Caenorhabditis nematodes. Aim #2: Determine how known co-factors interact with TRA-1 isoforms to regulate target genes. Aim #3: Identify new TRA-1 co-factors. Since TRA-1 is a model Gli protein, many of its co-factors and regulatory interactions are likely to be conserved. Thus, identifying co-factors and elucidating how they work with TRA-1 should illuminate human development and disease. In addition, novel co-factors that are not shared with humans could define new targets for antihelminthic drugs. Since preventing adult worms from reproducing in their human hosts is a critical for managing parasitic nematodes, the ability to target this part of the sex determination pathway could be invaluable. Finally, nematodes are one of the top models for sex determination (as shown by coverage in textbooks like Developmental Biology), so it is critical that we decipher the central part of the story.